This invention relates to compositions of alkyl aromatic aldehydes from a process for making alkyl aromatic aldehydes from the carbonylation of alkyl aromatic compounds.
Carbonylation of an alkyl aromatic compound to form an alkyl aromatic aldehyde can be carried out by a reaction generally referred to as the Gatterman Koch reaction. Published in 1897, Gatterman and Koch described the direct carbonylation of various aromatic compounds by the use of carbon monoxide (CO) and hydrogen chloride in the presence of aluminum chloride and cuprous chloride (Gatterman, L. and Koch, J. A., Chem. Ber., 30, 1622 (1897)). The reaction was subsequently expanded to include other Lewis acids. A review of such reactions is set forth in Olah, G. A., xe2x80x9cFriedel-Crafts and Related Reactionsxe2x80x9d, Wiley-Interscience, N.Y., Vol. 1153 (1964).
Catalysts used in a Gatterman-Koch carbonylation reaction are typically complexed with the resulting aromatic aldehyde product. To free the desired aldehyde from the acid catalyst, water is added and the resulting organic and aqueous phases separated. For example, water can be added to a tolualdehyde-AlCl3 complex to obtain the aldehyde product in a complex-free form. However, this separation procedure chemically alters and destroys the utility of the catalyst. This aqueous separation method, which leads to a one time use of catalyst, renders this process commercially unattractive as catalyst regeneration and recycle would be prohibitively expensive.
U.S. Pat. No. 2,485,237, describes replacing the hydrogen chloride and aluminum chloride catalyst with a hydrogen fluoride:boron trifluoride (HF:BF3) catalyst. An improved method of recovering the fluorides is described is U.S. Pat. No. 3,284,508.
A method to recycle the HF:BF3 was proposed by Olah, G. A. et al., J. Am. Chem. Soc., 98:1, 296 (1976). The carbonylation reaction is carried out at low temperatures, typically from 0xc2x0 C. to 20xc2x0 C, with excess HF. The lower boiling catalyst is separated from the aldehyde-catalyst complex by a distillation technique, condensed and returned to the carbonylation reactor. While this method is useful, it is generally desirable to have a method that avoids the use of HF, a material which requires special containment and handling facilities.
U.S. Pat. No. 3,948,998 describes a two-step process for making tolualdehyde. First, a toluene-HFxe2x80x94BF3 complex is formed and reacted with CO to form tolualdehyde. Second, additional CO and optionally additional toluene is added to the reaction medium. Other catalysts that have been used in a Gatterman-Koch type carbonylation include combinations of Lewis and strong Bronsted acids, e.g., SbF5xe2x80x94HF, described in U.S. Pat. No. 4,218,403. The use of Bronsted acids alone, such as fluorosulfonic acid or trifluomethane sulfonic acid, were also reported to be effective catalysts. See for example Olah, G. A. Laali, K., and Farooq, O., J. Org. Chem., 50, 1483 (1985).
However, the catalysts used in Gatterman-Koch carbonylation are typically complexed with the aldehyde product. Thus a stoichiometric amount of catalyst is xe2x80x9cconsumedxe2x80x9d in the reaction. Further, in order to obtain the aldehyde product in a complex-free form, a separation step is needed. For instance, water can be added to a tolualdehyde-AlCl3 complex to obtain the aldehyde product in a complex-free-form. However, this step chemically alters and destroys the utility of the catalyst. Such a separation, which leads to a one time use of the catalyst renders this process commercially unattractive as catalyst regeneration and recycle would be prohibitively expensive.
An aromatic aldehyde composition containing isomeric mixtures of dimethylbenzaldehydes prepared from contacting a mixed xylene feedstock with a carbonylation catalyst using a Gatterman-Koch type reaction. The aromatic aldehyde composition contains from about 80% to about 96% by weight, preferably from about 90% to about 96% by weight, 2,4-dimethylbenzylaldehyde, from about 2% to about 15% by weight, preferably from about 2% to about 6% by weight, 3,4-dimethylbenzylaldehyde, less than about 3% by weight 2,5 -dimethylbenzylaldehyde, and less than about 3% by weight 4-ethyllbenzylaldehyde. This aromatic aldehyde composition can then be oxidized using known methods to form an aromatic acid composition containing about 80% to about 98% by weight trimellitic acid and about 2% to about 10% by weight terephthalic acid.
The carbonylation catalyst used in the Gatterman-Koch type reaction is selected from perfluoroalkyl sulfonic acids with about 2 to about 18 carbon atoms, perfluoroether sulfonic acids with about 2 to about 18 carbon atoms, GaBr3, GaCl3, AlBr3, AlCl13, AlI3, TaF5, NbF5, or NbBr5. Preferably, the carbonylation catalyst is selected from perfluorohexane sulfonic acid, perfluorooctane sulfonic acid, and perfluoroethoxyethane sulfonic acid.
The invention is further directed to a composition of isomeric mixtures of tolualdehydes prepared from contacting a toluene feedstock with a carbonylation catalyst using a Gatterman-Koch type reaction. The composition contains about 85% to about 97% by weight para-tolualdehyde, about 2% to about 10% by weight ortho-tolualdehyde, and about 2% by weight or less, preferably about 1% by weight or less, more preferably about 0.5% by weight or less, meta-tolualdehyde. This aromatic aldehyde composition can then be oxidized using known methods to form an aromatic acid composition containing about 80% to about 98% by weight terephthalic acid, about 2% to about 10% by weight phthalic acid, and less than about 3% by weight isophthalic acid.
This invention is directed to compositional mixtures of alkyl aromatic aldehydes. The alkyl aromatic aldehyde compositions of the invention are produced from contacting an alkyl aromatic feed with a carbonylation catalyst. The alkyl aromatic aldehyde compositions can contain two or more of the following alkyl aromatic compounds: ortho-tolualdehyde, meta-tolualdehyde, para-tolualdehyde, 2,4-dimethylbenzaldehyde (2,4-DMB), 3,4-dimethylbenzaldehyde (3,4-DMB), 2,5-dimethylbenzaldehyde (2,5-DMB), 2,4,5,-trimethylbenzaldehyde, 4-ethylbenzaldehyde, 4-propylbenzaldehyde, and 4-isopropylbenzaldehyde. The tolualdehyde compositions are formed if toluene is used as the aromatic aldehyde feedstock. The dimethylbenzaldehydes and the higher 4-alkylbenzaldehydes are formed if a feedstock containing primarily mixed xylenes is used as a feedstock.
The alkyl aromatic aldehyde compositions can also contain a mixture of alkyl-substituted benzaldehydes, wherein the alkyl groups include, but are not limited to, are selected from methyl, ethyl, propyl, isopropyl, or butyl. These alkyl benzaldehyde compositions result from the reaction of an alkyl aromatic feedstock, containing at least three alkyl aromatic compounds, with carbon monoxide in the presence of a high boiling point carbonylation catalyst. The alkyl aromatic feedstocks that can be used to provide the alkyl-substituted benzaldehyde compositions include, but are not limited to, at least three of the following compounds selected from 1-methyl-3-propylbenzene, 1-methyl-2-propylbenzene, 1,4-diethylbenzene, 1-methyl-4-propylbenzene, butylbenzene, 2-ethyl-1,4 -dimethylbenzene, 4-ethyl-1,2-dimethylbenzene, 1-ethyl-2,4-dimethylbenzene, 1,2,4,5-tetramethylbenzene, and 1,2,3,5-tetramethylbenzene.
These alkyl aromatic feedstocks can be defined by an average boiling point of the alkyl aromatic feedstock. The average boiling point is defined by the summation of the wt % of each alkyl aromatic compound in the mixture multiplied by its respective boiling point. In one embodiment, the average boiling point of the alkyl aromatic feedstock is from about 140xc2x0 C. to about 170xc2x0 C., preferably from about 140xc2x0 C. to about 160xc2x0 C.
The alkyl aromatic aldehyde compositions can also contain a mixture of alkyl-substituted napthylaldehydes. These napthylaldehyde compositions result from the reaction of an alkyl-substituted napthalene containing feedstock, containing at least two alkyl-substituted napthalene compounds, with carbon monoxide in the presence of a high boiling point carbonylation catalyst. The alkyl-substituted napthalene feedstocks that can be used to provide the alkyl napthylaldehydes include at least two of the following compounds selected from 1-methylnapthalene, 2-methylnapthalene, 2,6-dimethylnapthalene, 2,7 -dimethylnapthalene, 1,3-dimethylnapthalene, 1,5-dimethylnapthalene, 1,6 -dimethylnapthalene, and 2-ethylnapthalene.
The alkyl aromatic compositions may comprise at least three of ethyldimethylbenzaldehye; methylpropylbenzaldehyde; diethyl-, propylethyl-, and trimethyl-benzaldehydes, trimethylbenzaldehyde, naphthaldehyde, diethylbenzaldehyde, and butylbenzaldehyde. One embodiment comprises about 32% to about 33% by weight ethyldimethylbenzaldehyde, about 18% to about 20% by weight methylpropylbenzaldehyde, about 20% to about 22% by weight diethyl- and propyl-ethylbenzaldehyde, about 18% to about 20% by weight trimethylbenzaldehydes, about 9% to 10% by weight napthaldehyde, and about 0.5% to about 1% by weight butylbenzaldehyde.
The alkyl aromatic aldehyde compositions may comprise at least two of methylnaphthaldehyde, dimethylnaphthaldehyde, naphthaldehyde, ethylnaphthaldehyde, indanecarboxaldehyde, and dimethylindanecarboxaldehyde. One embodiment comprises about 50% to about 52% by weight methylpropylaldehyde, about 25% to about 28% by weight dimethylnaphthaldehyde, about 11% to about 12% by weight naphthaldehyde, about 7% to about 8% by weight ethylnaphthaldehyde, and from about 5% to about 6% by weight indanecarboxaldehyde and dimethylindanecarboxaldehyde.
These alkyl-substituted napthalene feedstocks can be defined by an average boiling point of the alkyl-substituted napthalene feedstock. The average boiling point is defined by the summation of the wt % of each alkyl napthalene compound in the mixture multiplied by its respective boiling point. In one embodiment, the average boiling point of the alkyl-substituted napthelene feedstock is from about 170xc2x0 C. to about 220xc2x0 C., preferably from about 180xc2x0 C. to about 210xc2x0 C.
A select fraction of the alkyl aromatic aldehyde compositions, particularly the dialkyl and trialkyl aromatic aldehydes can be oxidized to the corresponding trimellitic acid and pyromellitic acid. These compounds can further undergo dehydration to form the corresponding anhydrides. Similarly, the para-tolualdehyde can be oxidized to terephthalic acid. The polycarboxylic acids are widely used as raw materials for alkyd resins, high grade plasticizer, and polyamide-imide polyester.
A xe2x80x9ccarbonylation catalystxe2x80x9d is defined as any compound or mixture of compounds that can catalyze the reaction of alkyl aromatic compounds with CO to form alkyl aromatic aldehydes. Generally, carbonylation catalysts are Lewis and/or Bronsted acids.
One embodiment of the invention is directed to an alkyl aromatic composition containing dimethylbenzaldehydes prepared from a mixed xylene feedstock using a Gatterman-Koch type reaction. The composition is obtained by reacting a mixed xylene feedstock with carbon monoxide in the presence of a carbonylation catalyst to form an aromatic aldehyde-catalyst complex. The aromatic aldehyde composition is then separated from the carbonylation catalyst. The catalyst can then be recycled to the carbonylation reaction.
This invention is also directed to a composition of isomeric mixtures of tolualdehydes prepared from a toluene feedstock using a Gatterman-Koch type reaction. The composition is obtained by reacting toluene with carbon monoxide in the presence of a carbonylation catalyst to form a tolualdehyde-catalyst complex. The tolualdehyde composition is then separated from the carbonylation catalyst by selectively volatilizing the tolualdehydes. The catalyst can then be recycled to the carbonylation reaction.
xe2x80x9cHigh boiling point carbonylation catalystxe2x80x9d means a carbonylation catalyst that has a boiling point that is higher than the desired aromatic aldehyde to be produced. Typically, the high boiling point carbonylation catalyst has a boiling point of at least about 210xc2x0 C., preferably at least about 230xc2x0 C., and more preferably at least about 250xc2x0 C. The high boiling point catalyst can be in liquid or solid form, the latter including supported and unsupported catalysts. Suitable support materials are, in general, well known in the catalyst art and include zeolites, ceramics and polymeric supports. A high boiling point carbonylation catalyst includes solid and supported catalysts that do not boil per se, but rather melt or degrade at high temperatures. These types of catalysts have a boiling point above their melting/degrading temperatures. So long as the aldehyde product can be distilled from the carbonylation catalyst, the unsupported catalyst and the solid or supported catalyst is within the scope of a high boiling point carbonylation catalyst. The term distilled is defined as separating the more volatile components of a mixture by the condensation of the vapors that are produced as the mixture is heated at atmospheric, sub-atmospheric, or super atmospheric pressure. Specific high boiling point carbonylation catalysts that can be used in the invention include perfluoroalkyl sulfonic acids having about 2 to about 18 carbon atoms, perfluoroether sulfonic acids having about 2 to about 18 carbon atoms, GaBr3, GaCl3, AlBr3, AlCl3, AlI3, TaF5, NbF5, and NbBr5.
The perfluoroalkyl sulfonic acids include compounds of the formula RfSO3H wherein Rf is a straight or branched chain perfluoroalkyl group having about 2 to about 18 carbon atoms, preferably about 2 to about 8 carbon atoms. Examples of such carbonylation catalysts include, but are not limited to, perfluoroethane sulfonic acid, perfluorobutane sulfonic acid, perfluorohexane sulfonic acid and perfluorooctane sulfonic acid.
The perfluoroether sulfonic acids are typically of the formula R1OR2SO3H wherein R1 and R2 each represent a straight or branched chain perfluoroalkyl group having about 1 to about 10 carbon atoms, preferably about 1 to about 4 carbon atoms. Acids of this type include compounds, or mixtures of compounds, of the formula CF3(CF2)zO(CF2)ySO3H wherein z and y are each independently a number from about 1 to about 3. An example of a perfluoroether sulfonic acid carbonylation catalyst is perfluoroethoxyethane sulfonic acid (y=2, z=1).
It should be understood that the carbonylation catalyst compounds described herein are used with their conventional meanings and thus include all variations of the compounds, including ionic and complexed forms, as can occur in situ. For example, in the presence of protons, the Lewis acid AlCl3 is believed to form H+ and AlCl4xe2x88x92. Further, the AlCl4xe2x88x92 can combine with AlCl3 to form Al2Cl7xe2x88x92 and higher homologues. Recognizing that in situ changes can occur to the xe2x80x9ccatalystxe2x80x9d compound, including complexing reactions or rearrangements all such forms and variations are collectively embraced by reference to the catalyst supplied to the reaction system. Thus, for example, identifying the catalyst as AlCl3 embraces carrying out the carbonylation reaction in the presence of any of AlCl3, AlCl4xe2x88x92, AlCl7xe2x88x92 and/or higher homologues thereof.
It is preferred that the carbonylation reaction be carried out in the absence of any added HF. This means that no effort is taken to add HF to the reaction system. However, HF can be present in the feeds as an impurity. Similarly, HF can be created in situ when a fluoride-containing compound is present. Such in situ formation does not correspond to xe2x80x9cadded HF.xe2x80x9d
The alkyl aromatic compounds that can be converted to the corresponding aromatic aldehydes in the invention include, but are not limited to, hydrocarbon aromatic ring compounds having one or more C1-C4 alkyl substituents. Generally the alkyl aromatic compounds are substituted benzenes having 1 to 3 alkyl groups. Examples of alkyl aromatic compounds include toluene, ortho- (o-), meta- (m-) and para- (p-) xylenes, ethylbenzene, pseudocumene (1,2,4-trimethylbenzene), mesitylene (1,3,5-trimethylbenzene), propylbenzene, and isopropylbenzene.
The alkyl aromatic compound is converted to the corresponding alkyl aromatic aldehyde as a result of the carbonylation reaction. The formyl group is directly bonded to the aromatic ring. The reaction of toluene with carbon monoxide (CO) under carbonylation conditions produces an aromatic aldehyde composition containing all three isomers; p-tolualdehyde, o-tolualdehyde, and m-tolualdehyde. Preferably, the aldehyde aromatic composition contains at least about 85% p-tolualdehyde, depending upon the catalyst, the reaction temperature, and reaction pressure. The o-tolualdehyde is produced in amounts from about 2% to about 10% by weight. The m-tolualdehyde is typically produced in amounts of about 2% by weight or less, preferably about 1% by weight or less, more preferably about 0.5% by weight or less.
The reaction of mixed xylenes with CO under carbonylation conditions of the invention produces three DMB isomers, that is, 2,4-DMB, 3,4-DMB, and 2,5-DMB, as well as 4-ethylbenzaldehyde. The product ratio 4-ethylbenzaldehyde:2,5-dimethylbenzene is typically from about 2:1 to about 1:3. The 4-ethylbenzaldehyde results from the reaction of 4-ethylbenzene, which is often, if not always present in mixed xylenes. Typically, mixed xylenes contain from about 10% to about 30% by weight 4-ethylbenzene.
Mixed xylenes are defined as an aromatic composition containing o-xylene, p-xylene, m-xylene, and 4-ethylbenzene. The mixed xylenes can also contain other hydrocarbons. Generally, these hydrocarbons will have a boiling point between about 120xc2x0 C. and about 160xc2x0 C. The mixed xylenes will contain from about 30% to about 75% m-xylene, from about 5% to about 30% o-xylene, from about 2% to about 20% p-xylene, and from about 5% to about 35% ethylbenzene.
FIG. 1 shows the relative product selectivity of a synthetic mixed xylene feed prepared without 4-ethylbenzene for the Lewis acid catalysts shown. Approximately, equal proportions of each isomeric xylene by weight were contained in the feed. As suggested by the isomeric product distribution of DMBs in FIG. 1, m-xylene is more reactive than o-xylene, which is more reactive than pxylene, under the carbonylation reaction conditions of the invention. The reactivity depends upon the carbonylation catalyst used. GaCl3 exhibits the highest selectivity to 2,4-DMB.
The carbonylation reaction is typically carried out by combining the carbonylation catalyst, optionally a solvent, a gas containing carbon monoxide, and the alkyl aromatic compound in a reactor. The carbonylation catalyst can be combined or dissolved with the alkyl aromatic compound to form a solution or a slurry. The latter is formed if a solid carbonylation catalyst is used. The reaction can be carried out in either batch, semi-batch, or continuous fashion.
The amount of carbonylation catalyst is not particularly limited and is generally equal to at least about one half of the molar amount of alkyl aromatic compound, typically from about 0.5 to about 20 times the molar amount of alkyl aromatic compound. For a Bronsted acid catalyst such as the perfluoroalkyl sulfonic acids, it is preferred that the catalyst be provided in molar excess to the alkyl aromatic compound, preferably from about 6 to about 20 molar excess, more preferably from about 6 to about 12 molar excess of carbonylation catalyst. In general, higher Bronsted acid catalyst concentrations provide for higher conversion. For a Lewis acid catalyst such as aluminum halides and gallium halides, it is preferred that the carbonylation catalyst be provided in molar amounts of about 0.08 to about 1.5 times the molar amount of alkyl aromatic compound. Preferably, the Lewis acid catalyst is provided in about or slightly in excess of about a 1:1 molar ratio to the alkyl aromatic compound.
The carbonylation reaction is generally carried out at a pressure from about 0 kg/cm2 to about 300 kg/cm2 (gauge), preferably from about 15 kg/cm2 to about 200 kg/cm2 (gauge), more preferably from about 4 kg/cm2 to about 100 kg/cm2 (gauge), and most preferably from about 4 kg/cm2 to about 25 kg/cm2 (gauge). Generally, an increase in pressure increases the cost of the reaction and/or the equipment and must be balanced against the increased productivity, if any. The use of a lower reaction pressure can be facilitated by incorporating co-catalysts, such as copper oxide or silver oxide into the reaction mixture. These co-catalysts can be used to improve the conversion rate at lower reaction pressures and/or lower temperatures.
Specifically, cuprous chloride, as was used in the original Gatterman-Koch reaction, copper oxide or silver oxide, as are described in U.S. Pat. No. 4,518,798 can each be used to improve the conversion rate at lower reaction pressures and/or more mild overall reaction conditions. Other metal salts as are known in the art for carbonylation can also be used.
The presence of CO2 or H2 in the carbon monoxide gas does not generally affect the carbonylation reaction. Accordingly, synthesis gas, which is comprised of CO, H2, and optionally CO2 in varying proportions, can be used as a source of CO. A stoichiometric excess of CO is generally introduced to the reactor. The CO partial pressure in the reactor is from about 0 kg/cm2 to about 200 kg/cm2 (gauge), preferably from about 1 kg/cm2 to about 100 kg/cm2 (gauge), and more preferably from about 2 kg/cm2 to about 25 kg/cm2 (gauge). The amount of CO is generally at least about 20 mol % of the gas supplied. For example, synthesis gas can vary from a CO:H2 molar ratio of about 1:1 to about 1:3. Further CO2 can also be present in amounts of up to about 30 mol %. Of course, the gas supplied to the reactor can be 100% CO.
The carbonylation reaction can be carried out over a wide range of temperatures. Usually the reaction temperature is within the range of from about 0xc2x0 C. to about 175xc2x0 C. more typically within the range of from about 0xc2x0 C. to about 100xc2x0 C. such as from about 0xc2x0 C. to about 50xc2x0 C. The temperature of the carbonylation reaction will depend upon the reaction conditions, the alkyl aromatic feed, the desired aromatic aldehyde composition, and the carbonylation catalyst used.
The carbonylation reaction is carried out for a sufficient time to achieve the desired product or conversion under the conditions employed. Generally the reaction is run for about 0.1 to about 5 hours although longer or shorter times can be used depending upon the reaction conditions, the alkyl aromatic feed, the desired aromatic aldehyde composition, and the carbonylation catalyst used.
In one embodiment, which takes advantage of the relatively high boiling point of the carbonylation catalyst, the aromatic aldehyde composition is separated from the resulting aldehyde-catalyst complex by selectively volatilizing the aldehyde composition from the aldehyde-catalyst complex. The aldehyde composition is volatized without substantial degradation. It is preferred that no more than about 30% by weight, more preferably no more than about 10% by weight, of the aldehyde composition resulting from the carbonylation reaction degrade during the selective volatilization separation.
The volatilization technique used should achieve separation in a short time period in order to avoid unwanted side reactions and/or degradation that are prone to occur in heating the aromatic aldehyde-catalyst complex over an extended period at high temperatures. Generally, the volatilization technique has a liquid residence time of less than about 5 minutes, preferably less than about 3 minutes, more preferably less than about 1 minute. Suitable techniques include evaporation, vaporization, flash distillation and combinations thereof. As is well understood, increasing the temperature and/or decreasing the pressure will increase the rate of volatilization. Desirably, the temperature used to volatilize the desired aldehyde is from about 90xc2x0 C. to about 350xc2x0 C., preferably from about 90xc2x0 C. to about 250xc2x0 C., more preferably from about 90xc2x0 C. to about 200xc2x0 C.
The term xe2x80x9cselectively volatilizingxe2x80x9d means that the volatilizing technique preferably volatilizes the desired aromatic aldehyde, and little, if any, of the high boiling point carbonylation catalyst. This leads to an aromatic aldehyde-rich vapor and a carbonylation catalyst-rich liquid. However, a perfect separation of the aromatic aldehyde from the high boiling point carbonylation catalyst is generally not possible because the high boiling point carbonylation catalyst is likely to have some vapor pressure. As a result, the aromatic aldehyde-rich vapor will contain some amount of high boiling point carbonylation catalyst. Accordingly, for purposes of the invention, the separation is considered to be selective for the aromatic aldehyde if less than about 50%, preferably less than about 30%, and more preferably less than about 15%, of the high boiling point carbonylation catalyst is present in the resulting aromatic aldehyde-rich vapor.
One way to selectively volatilize the aromatic aldehyde is to use a wiped-film evaporator, sometimes referred to as an agitated film evaporator. This volatilization unit is generally comprised of a straight or tapered tube containing concentric, rotating paddles. The edge of the paddles can be in or above the film layer. The liquid, aromatic aldehyde-catalyst complex is directed to the interior surface of the tube as a thin film. The paddles are rotated to aid in the formation of the desired film thickness. The wall of the tube is normally heated. The volatilized aromatic aldehyde is directed to the annular region of the tube and removed as a vapor. The process can be run at a variety of pressures, but is preferably carried out under reduced pressure. The wall temperature is typically at least about 90xc2x0 C., and is usually in the range of about 90xc2x0 C. to about 350xc2x0 C. It should be noted that the lower temperatures, i.e. those less than about 200xc2x0 C., are sufficient to volatilize the aromatic aldehyde provided the pressure is sufficiently low. To increase the separation efficiency, regardless of the temperature, it is preferred that the pressure is less than or equal to about 0.5 kg/cm2 (gauge), more preferably less than about 0.1 kg/cm2 (gauge). The condensate contains the aromatic aldehyde and unreacted alkyl aromatic compounds while the majority or all or substantially all of the carbonylation catalyst remains in the liquid film. The high boiling point carbonylation catalyst can then be recycled to the carbonylation reactor.
Another way to selectively volatilize the aromatic aldehyde is to use a flash distillation unit. The aromatic aldehyde-catalyst complex from the carbonylation reactor is sent to a flash chamber where a portion of the aromatic aldehyde product is volatilized. The majority or all or substantially all of the high boiling point carbonylation catalyst remains in the liquid phase and exits as a bottom stream. The high boiling point carbonylation catalyst can then be recycled to the carbonylation reactor. The vapor or distillate containing the aromatic aldehyde product and unreacted alkyl aromatic compounds is condensed. The aromatic aldehyde is then further separated from the unreacted aromatic compounds. The unreacted aromatic compounds can then be recycled to the carbonylation reactor.
As previously stated, it is preferred that the separation of the aromatic aldehyde from the high boiling point carbonylation catalyst occur relatively rapidly so as to avoid degradation of the desired aromatic aldehyde product. One way this can be achieved is to heat the liquid phase in a heat exchanger very quickly just prior to introducing the liquid, aromatic aldehyde-catalyst complex to the flash chamber. The temperature is preferably increased to at least about 200xc2x0 C., more preferably within the range from about 230xc2x0 C. to about 300xc2x0 C., in less than about 4 minutes, preferably less than about 2 minutes, more preferably in less than about 20 seconds. The rapidly heated complex is then supplied to the flash chamber where the more volatile aromatic aldehyde products and unreacted aromatic compounds are separated from the high boiling point carbonylation catalyst. Under these conditions, the aromatic aldehyde will begin to vaporize in the heat exchanger before reaching the flash chamber, thereby reducing the average aldehyde-catalyst contact time at the higher temperatures in the flash chamber. The residence time of the complex in the flash chamber is typically about 10 seconds or less, preferably about 5 seconds or less, for the liquid and generally about 5 seconds or less, preferably about 5 seconds or less, for the volatilized compounds.
After the flash volatillization, the vapor is preferably subjected to an absorbing tower or other suitable unit to remove any remaining high boiling point carbonylation catalyst in the vapor. For example, the vapor can be run through a multi-plate column where a diluent, e.g. toluene, is added. Preferably, the diluent is added in a counter-current direction. The high boiling point carbonylation catalyst, if any, will re-complex with the aromatic aldehyde and rapidly condense out of the vapor. This leaves a vapor with the desired aromatic aldehyde and excess diluent and/or aromatic alkyl compound with essentially no high boiling point carbonylation catalyst. The liquid stream containing the aldehyde-catalyst complex can be recycled back to the heat exchanger and flash chamber. Such an absorbing column is preferably used with the flash separation technique, but is suitable for use with any selective volatilization method.
The separated alkyl aromatic aldehydes can then be oxidized to form the corresponding aromatic acids using methods known in the art. The aromatic acids can then be dehydrated to the corresponding anhydrides using methods known in the art. In general, oxidation comprises combining the aromatic aldehyde with oxygen or an oxygen containing gaseous mixture, optionally in the presence of an oxidation catalyst. A solvent such as a lower aliphatic acid, an ester, or water is typically used in an oxidation reaction. Examples of solvents include, but are not limited to, formic acid, acetic acid, propionic acid, n-butyric acid, iso-butyric acid, valeric acid, trimethylacetic acid, caproic acid, methyl benzoate, dimethyl terephthalate, and water.
The oxidation catalysts are well known and include cobalt salts, manganese salts, chromium salts, lanthanide salts especially cerium salts, and mixtures thereof. Examples of catalysts include cobalt acetate or naphthenate, and manganese acetate or naphthenate. A combination of Co/Mn is particularly preferred as a catalyst. The amount of catalyst is not particularly limited and is generally within the range from about 50 ppm to about 1000 ppm for managese and about 50 ppm to about 2000 ppm for cobalt, based on the solvent. Bromine or other free radical initiators can optionally be included to aid in the reaction. However, because the oxidation is carried out with an aldehyde, free radical initiators such as HBr can be advantageously minimized or omitted. Further, in view of its corrosive nature, bromine, or progenitor thereof, is preferably excluded from the oxidation reaction or minimized as is described in U.S. Pat. No. 5,453,538.
Oxygen used in the oxidation reaction can be supplied to the reactor as pure oxygen or as a mixed gas containing other inert gases such as nitrogen. Thus, air can be used as an oxidizing gas. The oxidation reaction is preferably conducted at a pressure from about 0 kg/cm2 to about 35 kg/cm2 (gauge), more preferably from about 10 kg/cm2 to about 30 kg/cm2 (gauge). The oxidation reaction temperature is generally within the range from about 100xc2x0 C. to about 252xc2x0 C., more typically from about 120xc2x0 C. to about 240xc2x0 C.
The aromatic aldehyde compositions of the invention can be converted to the corresponding acids or anhydrides. For example, p-tolualdehyde produced from the carbonylation of toluene can be oxidized to terephthalic acid. The 4 -ethylbenzaldehyde produced in the carbonylation of the mixed xylenes can also be oxidized to terephthalic acid.
Once separated from the produced 4-ethylbenzaldehyde, the dimethylbenzaldehydes produced from the mixed xylenes can be oxidized to form trimellitic acid. The trimellitic acid can then be dehydrated to form trimellitic anhydride. Thus, trimellitic anhydride can be produced from a mixed xylene feed without the need to isolate a particular isomer of the mixed xylene feed.
Alternatively, the aromatic aldehyde product from the carbonylation of mixed xylenes can be directed to an oxidation unit without prior separation of 4-ethylbenzaldehyde. The resulting aromatic acid composition should contain about 80% to about 98% by weight trimellitic acid from the oxidation of the dimethylbenzaldehydes and about 2% to about 10% by weight terephthalic acid from the oxidation of 4-ethylbenzaldehyde.
Other alkyl aromatic aldehydes produced according to the invention can also be oxidized to form the corresponding aromatic acids. For example, pyromellitic acid can be produced from 2,4,5-trimethylbenzaldehyde which is obtained by carbonylating pseudocumene. The pyromellitic acid can then be dehydrated to form pyromellitic dianhydride. Likewise, mesitaldehyde can be oxidized to form trimethyl benzoic acid.
The following non-limiting examples are provided in order to further demonstrate the various embodiments and advantages of the invention.